1. Field of the Invention
The present invention relates to a rotation table that is mounted on a machining table of a machine tool.
2. Description of the Related Art
In recent years, a rotation table has been widely used in a machine tool. In many cases, the rotation table is particularly used for an indexing operation, and the indexing operation and the reliability of a clamping mechanism become important factors of the rotation table.
As a technique of an air purging mechanism of the rotation table, Japanese Patent Application Laid-Open No. 11-267939 discloses a technique in which purging air is partly supplied from air used in a clamping mechanism and air is discharged to the outside of a casing by an adjustment valve attached to the casing at the lower portion thereof when the pressure inside the rotation table becomes a predetermined pressure higher than the atmospheric pressure. Thus, the pressure inside the rotation table is normally maintained in a predetermined pressure higher than the atmospheric pressure.
FIG. 1 is a schematic diagram illustrating the entire configuration of a machine tool.
A rotation table 3 is used in a state where it is mounted on a machining table 2 disposed on a bed 7 of a machining center 1. In many cases, the rotation table 3 is exposed to a cutting fluid 5 ejected from a coolant nozzle 4. For that reason, there is a possibility that the cutting fluid 5 may intrude into the rotation table 3 so that electric parts or mechanical parts inside the rotation table are damaged. In order to prevent the damage, the pressure inside the rotation table 3 is maintained at a predetermined pressure (an air purging pressure) higher than the atmospheric pressure.
Incidentally, in a pneumatic drive type clamping mechanism mounted on the rotation table 3, a piston 6 is generally driven by air as illustrated in FIG. 2.
As illustrated in FIG. 2, the pneumatic drive type clamping mechanism is formed by two pressure chambers 22a and 22b respectively used for the clamping and unclamping operations through the piston 6. When air flows into the clamping pressure chamber 22a and the other unclamping pressure chamber 22b is released into the rotation table 3, the piston 6 moves toward the unclamping pressure chamber 22b. When the piston 6 is operated in the reverse direction, air flows into the unclamping pressure chamber 22b, and an air flow passage 26 is switched by an electromagnetic valve 8 so that the clamping pressure chamber 22a is released into the rotation table 3.
In this way, in the general rotation table 3, air is discharged into the rotation table 3 with the operation of the piston (see electromagnetic valve discharge ports 8a and 8b). For this reason, there is a case in which the pressure chambers released into the rotation table may communicate.
An example of an air purging structure is illustrated in FIG. 2. A supply port 26a is formed in the course of the passageway 26 so that purging air is partly supplied from the air used to drive the clamping mechanism. The supply port 26a is provided with a needle valve 9 which adjusts the amount of air flowing into the rotation table. Further, a drain hole 28 is formed in the casing 10 so as to release air supplied for air purging, and the drain hole 28 is also provided with a needle valve 11, thereby adjusting an outflow amount of the air.
The air purging pressure is generally about 0.01 MPa, but the air treated by the pneumatic drive type clamping mechanism is generally high so as to be 0.5 MPa. For this reason, when air is once discharged into the rotation table 3 by the clamping/unclamping operation, the pressure inside the rotation table 3 increases. Since the inside of the rotation table 3 is sealed due to the above-described reason, the pressure inside the rotation table 3 does not decrease promptly. When the clamping or unclamping operation is performed while the pressure inside the rotation table 3 is higher than the air purging pressure, the pressure chamber released into the rotation table communicates with the inside of the rotation table, and hence a phenomenon occurs in which the operation of the piston 6 becomes slower than the normal operation. When the clamping/unclamping operation is frequently performed in a short period of time, this problem becomes severe. When the clamping/unclamping operation is slow in the rotation table, the clamping mechanism may be damaged when a detection structure does not exist.